1. Field of the Invention
The Present invention relates to a semiconductor device, and more specifically, it relates to a method for manufacturing a semiconductor device having a capacitor such as DRAM (dynamic random access memory).
2. Description of the Prior Art
A DRAM in which a memory is constituted of one transistor and one capacitor has been highly integrated by the miniaturization of memory cells.
With the decrease of a memory cell area, an occupied area of a capacitor decreases, so that it is difficult to secure a stored charge capacity (about 25 fF) required for soft error resistance.
At present, a storage electrode called a stack type has been widely used, and a technique has been used in which fine hemispherical silicon crystal grains, i.e., an HSG-Si (hemispherical grain-silicon) is formed on the surface of the storage electrode to increase a virtual surface area of the electrode.
The formation of this HSG-Si can be accomplished by annealing a clean amorphous silicon film surface in an amorphous-crystalline transition temperature range to form crystal nuclei on the amorphous silicon film surface, and then allowing crystals to grow by the surface migration of a silicon atom.
Therefore, the amorphous silicon film surface which is a parent body is required to be a clean surface which is free from a spontaneous oxide layer and organic contaminants, and an annealing atmosphere at the formation of the HSG-Si is also required to be a high vacuum atmosphere or a non-oxidizing atmosphere.
Furthermore, the surface of crystallized silicon is more stable than that of amorphous silicon, and on the surface of crystallized silicon, the surface migration of the silicon atom scarcely occurs, so that any HSG-Si is not formed. Therefore, the surface on which the HSG-Si is formed must be the surface of amorphous silicon.
Techniques for forming the HSG-Si can be classified into a selective HSG method and a blanket HSG method in accordance with a film formation principle, and these methods have the following merits and demerits.
The selective HSG method is a method in which the HSG is selectively formed only on the surface of a previously formed storage electrode, and after the film formation, an etch back step is not necessary in contrast to the blanket HSG method. Hence, the selective HSG method has a merit that the number of steps is small.
Japanese Patent Application Laid-open No. 315543/1993 has suggested a method which comprises patterning a storage electrode by the use of the selective HSG method, depositing amorphous silicon, carrying out the etch back to separate electrodes from each other, and then forming the HSG-Si.
Furthermore, in the selective HSG method, the shape of the storage electrode is not restricted by the etch back step, and therefore, the selective HSG method has a merit that it is also applicable to the storage electrode having an intricate shape such as a cylinder type or a fin type.
However, the HSG formation by the selective HSG method depends very sensitively on the state on an electrode surface, and particularly, it has a problem that there easily occurs a fault that any HSG is not formed owing to the presence of a spontaneous oxide layer and contamination with organic substances.
On the contrary, the blanket HSG method is a method in which the HSG is formed all over, as denoted by its name. In the first place, an amorphous silicon film which is a parent body is formed all over, and annealing is successively carried out without exposing the amorphous silicon film to the atmosphere to form the HSG.
Accordingly, the blanket HSG method has a merit that any spontaneous oxide layer and any contamination with organic substances do not take place. However, owing to the overall film formation, the storage electrodes are required to be separated from each other by a technique such as the etch back, and the shape of the electrodes are restricted by the etch back step.
Next, a conventional technique will be described mainly in accordance with the blanket HSG method with reference to FIGS. 3(a) to 3(e).
Firstly, as shown in FIG. 3(a), a contact hole is formed in a drain 2 of a semiconductor substrate incorporated with an MOS-FET and the like, and a first silicon film 9 containing phosphorus as an impurity is then formed in an amorphous state by the use of an already known vacuum chemical vapor growth method.
Next, as shown in FIG. 3(b), the first silicon film 9 is worked into a desired shape by an already known photoetching technique to form a part of a storage electrode. In FIG. 3(b), a single storage electrode is only shown, but in fact, this is formed in the form of dumbbells.
Afterward, a spontaneous oxide layer on the first silicon film 9 is removed therefrom with diluted hydrofluoric acid or the like, and as shown in FIG. 3(c), a second silicon film 15 is formed all over from a gas system containing silane (SiH.sub.4) or disilane (Si.sub.2 H.sub.6) by the use of a vacuum chemical vapor growth method. At this time, film formation conditions are set so that the second silicon film 15 may be in an amorphous state.
Successively, annealing is carried out in a high vacuum atmosphere or a non-oxidizing atmosphere without exposing the second silicon film 15 to the atmosphere, thereby allowing hemispherical silicon crystal grains 13 to grow on the surface of the second silicon film 15, as shown in FIG. 3(d).
Afterward, etch back is done by the use of an already known anisotropic dry etching technique to separate storage electrodes from each other, as shown in FIG. 3(e).
Next, a dielectric film and a plate electrode are formed to prepare a capacitor (not shown).
The blanket HSG method is insensitive to the spontaneous oxide layer and the organic contaminants on the surface of the storage electrode, and it has a wide process margin. Hence, the blanket HSG method is considered to be an excellent method.
In the blanket HSG method, however, a fault that the storage electrode is not partially converted into the HSG or does not grow to a sufficient size tends to take place with the increase in the film thickness of the first silicon film 9.
The storage electrode which is not converted into the HSG is short of a capacity, so that it has drawback that a normal operation is impossible and a bit failure occurs.
According to the results of inspection, it has been elucidated that a part of the first silicon film 9 is crystallized, whereby the second silicon film 15 is also crystallized prior to the growth of the HSG-Si. As discussed above, any HSG-Si is not formed on the crystallized surface.
The reason why the occurrence of the fault increases with the increase in the film thickness of the first silicon film 9 is that a heat history is prolonged by the prolongation of a film formation time, so that crystal nuclei are easily formed in the film or between the film and an undercoat.
It can be considered that since the velocity of the crystal growth in the film is higher than the film formation velocity of amorphous silicon, the crystallization reaches the surface.
In the conventional technique, electrical connection is considered to be important, and hence, after the removal of the spontaneous oxide layer on the surface of the storage electrode, the second silicon film 15 is formed. Therefore, as shown in FIG. 4, it can be presumed that when the first silicon film is crystallized, this fact further leads to the crystallization of the second silicon film 15.
Thus, by treating the surface with a mixed solution of ammonia and aqueous hydrogen peroxide prior to the formation of the second silicon film 15, and then covering the surface with a dense spontaneous oxide layer in order to shut out the influence by the crystallization of the first silicon film 9, the fault that the HSG is not obtained has been remarkably reduced.
On the other hand, there occurs another problem that an impurity is not sufficiently fed to the formed HSG, so that a capacity cannot be increased as expected owing to depletion.
The formed HSG-Si does not contain any impurity, so long as it is not subjected to any treatment. Therefore, a certain means should be taken to introduce the impurity thereinto.
In the conventional technique, the impurity in the first silicon film 9 is thermally diffused into the HSG-Si through the second silicon film 15, but it has been apparent that the spontaneous oxide layer which is formed prior to the formation of the second silicon film 15 functions as a barrier or a trap of the impurity diffusion, so that the impurity is not sufficiently fed to the HSG-Si.
Thus, in order to easily feed the impurity to the HSG-Si, it was attempted that the second silicon film 15 was doped with the impurity and the HSG-Si is then formed. In this case, however, the growth rate of the HSG-Si was low, so that the sufficiently large HSG-Si could not be obtained. FIG. 5 shows a relation between an annealing time at 565.degree. C. and the grain size of the HSG-Si.
When the size of the HSG-Si is regulated to, for example, 70 nm, an annealing time of about 50 minutes is necessary in the case of doped amorphous silicon of 2.03E20 [atoms/cc] (a curve x in FIG. 5), though an annealing time of about 5 minutes is enough in the case of non-doped amorphous silicon (a curve .cndot. in FIG. 5).
In the case that phosphorus is contained as the impurity in the amorphous silicon film which is the parent body, the surface migration of a silicon atom is determined by the elimination of phosphorus, and the higher the concentration of the impurity is, the lower the growth rate of the HSG-Si is.
On the contrary, the crystals in the film grow more easily when the impurity concentration is high, and if the crystals in the second silicon film 15 grow and reach the surface before the HSG-Si grows sufficiently, the growth of the HSG-Si stops. Therefore, even if the annealing time is merely prolonged, the large grains cannot always be obtained.
As described above, by the conventional techniques, it has been difficult that the impurity is sufficiently introduced into the HSG-Si, while the HSG-Si formation failure due to the crystallization is restrained, to obtain a high capacity value.